Fe-rich smectite is ubiquitous in soil environments and closely linked to the fate and mobility of hazardous trace metals and particularly to the variations in the biogeochemical redox reactions of structural Fe that determine the sorption and desorption properties of clay minerals. The biotic/abiotic redox reactions of a Fe-rich smectite, nontronite (NAu-1), were performed at various reaction times using the Fe-reducing bacterium Shewanella oneidensis MR-1 at 30°C and Na-dithionite (Na2S2O4), respectively. The extent of biotic Fe-reduction of NAu-1 after 30 days of incubation reached up to 10.7% of total Fe and the range of abiotic Fe-reduction varied from 4.9–46.6% at reaction times of 5 min, 30 min, 1 h, and 4 h. The biotically and abiotically Fe-reduced NAu-1 samples were spiked with Pb concentrations of 0.07, 0.2, 0.5, and 1.0 mg/kg and incubated under aerobic or anaerobic conditions for 24 h.

The amounts of Pb in the supernatants were analyzed using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Multi-collector (MC)-ICP-MS. The amounts of Pb removed from the supernatants were negatively related to the extent of Fe(III) reduction in the abiotically Fe-reduced NAu-1 samples. In contrast, less Pb (~15%) was removed from the biotically Fe-reduced NAu-1 samples with a similar extent of Fe(III) reduction. Changes in the isotopic 208/204Pb ratio indicated that the lighter 204Pb isotope was preferentially adsorbed to the NAu-1 samples with less Fe reduction and indicated that variations in the net layer charge affected isotopic fractionation. Significant differences in the 208/204Pb ratios for NAu-1 samples that were biotically Fe-reduced under anaerobic conditions were measured and indicate that the reversibility of the structural/chemical modifications that occur under redox conditions can affect Pb removal and, thus, isotope fractionation. These results collectively infer that the biogeochemical properties of clay minerals should be considered in order to understand the fate of trace metals in natural environments.

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